1. Field of the Invention
The present invention relates to a focus detection apparatus, an image pickup device, and an electronic camera, and more particularly to a focus control technique for a structure in which a focus detection system using a phase-difference detection method is installed in an image pickup device.
2. Description of the Related Art
A contrast detection method and a phase-difference detection method have been used as a focus detection technique for an electronic camera or the like. In the contrast detection method, an image signal output from an image pickup device is used to evaluate and determine the sharpness of an object image formed by an image-pickup optical system by using a predetermined function. The position of the image-pickup optical system on an optical axis is adjusted such that the function value approaches an extreme value. In the phase-difference detection method, two object images are formed by respective light beams that pass through different portions of an exit pupil of the image-pickup optical system. A positional phase difference between the two object images is detected, and is converted into an amount of defocus of the image-pickup optical system. According to the phase-difference detection method, since the amount of defocus can be determined, a focused state can be established in a shorter time compared to the contrast detection method in which the evaluation function value is determined while gradually moving the lenses of the image-pickup optical system along the optical axis.
According to the phase-difference detection method of the related art, a light beam that passes through the image-pickup optical system is divided into two beams by a condenser lens, and the thus-obtained two beams are caused to pass through respective separator lenses so that the beams are focused on a detection device (other than the image pickup device). This structure can be omitted by installing a focus detection system that uses the phase-difference detection method in the image pickup device itself (see, for example, Japanese Unexamined Patent Application Publications Nos. 2001-83407 (FIG. 18) and 2003-244712 (FIG. 2)).
For example, Japanese Unexamined Patent Application Publication No. 2001-83407 (FIG. 18) proposes a technique in which a photoelectric converter (photodiode) of each pixel is divided into two photoelectric converter parts which receive light beams that pass through an exit pupil of an image-pickup optical system at different areas of the exit pupil. Charge signals obtained by the two photoelectric converter parts of each pixel are compared with charge signals in a focused state that are obtained in advance. Thus, the focus detection using the phase-difference detection method is performed.
Japanese Unexamined Patent Application Publication No. 2003-244712 (FIG. 2) proposes a technique in which a photoelectric converter is not divided in pixels that are provided with a color filter and used to capture an image, but is divided into two photoelectric converter parts in pixels that are provided with no color filter and used for the focus detection. In the process of reading image signals in an image capturing operation, only the charge signals from the pixels provided with the color filter are used. In the process of focus detection, only the charge signals from the pixels provided with no color filter are used. Thus, the focus detection using the phase-difference detection method is performed.